A solution containing both Fe
3+
and Al
3+
can be selectively analyzed for Fe
3+
by
buffering to a pH of 2 and titrating with EDTA. Th
e pH of the solution is then raised
to 5 and an excess of EDTA added, resulting in the
formation of the Al
3+
- EDTA
complex. The excess EDTA is then back-titrated usi
ng a standard solution of Fe
3+
,
providing an indirect analysis for Al
3+
.
a.
Show that at a pH of 2, the formation of t
he Fe
3+
- EDTA complex is
favorable, while the formation of the Al
3+
- EDTA complex is not.

The percent (% w/w) water in a freeze-dried antibiotic is determined
in the following manner. A 0.175-g sample is placed in a vial along with 4.489
g of methanol. Water in the sample extracts into the methanol, which is then
analyzed. If GC analysis of the methanol yields a peak height of 13.66, what is
the % w/w water in the antibiotic?